Afghanistan’s rapidly-advancing Taliban insurgents have entered a western provincial capital only hours after taking the country’s second and third largest cities in a lightning advance just weeks before America is set to end its longest war.

The capture of Kandahar and Herat represent the biggest prizes yet for the Taliban, who have taken 12 of Afghanistan’s 34 provincial capitals as part of a week-long blitz.

While Kabul itself is not directly under threat yet, the losses and the battles elsewhere further tighten the grip of a resurgent Taliban, who are estimated to now hold more than two-thirds of the country and continue to press their offensive.

A map showing areas now controlled by Taliban as of Friday (AP)

With security rapidly deteriorating, the United States planned to send in 3,000 troops to help evacuate some personnel from the US Embassy in Kabul.

Separately, Britain said some 600 troops would be deployed on a short-term basis to support British nationals leaving the country, while Canada is sending special forces to help evacuate its embassy.

Thousands of Afghans have fled their homes amid fears the Taliban will again impose a brutal, repressive government, all but eliminating women’s rights and conducting public executions. Peace talks in Qatar remain stalled, though diplomats are still meeting.

Fazel Haq Ehsan, chief of the provincial council in the western Ghor province, said on Friday that the Taliban had entered Feroz Koh, the provincial capital, and that fighting was continuing in the city.

The Taliban also claimed to have captured Qala-e Naw, capital of the western Badghis province. There was no official confirmation.

Taliban fighters patrol inside the city of Ghazni, south-west of Kabul (Gulabuddin Amiri/AP)

The latest US military intelligence assessment suggests Kabul could come under insurgent pressure within 30 days and that, if current trends hold, the Taliban could gain full control of the country within a few months.

The Afghan government may be forced to pull back to defend the capital and just a few other cities in the coming days if the Taliban maintain momentum.

The onslaught represents a stunning collapse of Afghan forces after the US spent nearly two decades and 830 billion dollars (£600 billion) trying to establish a functioning state after toppling the Taliban in the wake of the September 11 attacks in 2001.

The advancing Taliban ride on American-made Humvees and carry M-16s pilfered from Afghan forces.

Afghan security forces and the government have not responded to repeated questions from journalists, instead issuing video communiques that downplay the Taliban advance.

Taliban fighters in Ghazni (Gulabuddin Amiri/AP)

Bill Roggio, a senior fellow at the Foundation for Defence of Democracies, said the Afghan army had rotted from within due to corruption and mismanagement, leaving troops in the field poorly equipped and with little motivation to fight.

The Taliban, meanwhile, have spent a decade taking control of large areas of the countryside, positioning themselves to rapidly seize key infrastructure and urban areas once President Joe Biden announced the US withdrawal.

The difficulty of moving troops out to the provinces means the government is likely to focus all its efforts on defending the capital.

In Herat, Taliban fighters rushed past the Great Mosque in the historic city — a structure that dates to 500 BC and was once a spoil of Alexander the Great — and seized government buildings. Witnesses described hearing sporadic gunfire at one government building while the rest of the city fell silent under the insurgents’ control.

Herat had been under militant attack for two weeks, with one wave blunted by the arrival of warlord Ismail Khan and his forces. But on Thursday afternoon, Taliban fighters broke through the city’s defensive lines and later said they were in control.